From the current version of slab-alloc from cargo clean before each compilation:
Summary
Compilation difference: 2.5% slower
Runtime difference: 5.5% slower
Commands
(output trimmed)
$ time cargo test --release --no-run
real 1m30.323s
user 1m32.598s
sys 0m5.703s
$ time cargo test --release
real 0m8.650s
user 0m13.695s
sys 0m5.665s
$ RUSTFLAGS='-C codegen-units=16 -Z thinlto' time cargo test --release --no-run
real 1m32.65s
user 3m4.69s
sys 0m8.18s
$ RUSTFLAGS='-C codegen-units=16 -Z thinlto' time cargo test --release
real 0m9.15s
user 0m15.88s
sys 0m5.53s